The present invention is generally directed to a rear-wheel steer control device which steers rear wheels in addition to front wheels, depending on vehicle motion conditions, for improving vehicle stability and so on. More particularly, the present invention is directed to reduce man-hours required for setting a target rear-wheel steering angle of the device.
It is well known that some vehicles possess a rear-wheel steering angle control device which steers rear wheels in addition to front wheels, depending on vehicle motion conditions, for improving vehicle stability and so on. In such a rear-wheel steer control device, a question is raised as to how to cope with setting an optimal target steering angle relative to a front steering angle depending on various vehicle motion conditions. To solve this question, a publication whose title is xe2x80x98Steering System and Steering Stability for Vehiclesxe2x80x99 and which is published as the first version by SANKAIDO publishing on Sep. 10, 1996 provides an idea on page 196, titled xe2x80x9817.5.3 4WS control for zeroizing side skidxe2x80x99 which says that while the vehicle-body slip angle (i.e. a deviation angle between the vehicle travel direction and the vehicle lengthwise direction) is focused, controlling the rear-wheel steering angle continually to reducing the vehicle slip angle to zero makes it possible to obtain an ideal vehicle dynamics with excellent vehicle stability.
However, conducting a road test according to the teaching of the aforementioned xe2x80x984WS control for zeroizing side skidxe2x80x99 reveals that the gain of the target rear-wheel steering angle relative to the front-wheel becomes too large, thereby giving an uncomfortable feeling to an ordinary driver. Thus, the aforementioned xe2x80x984WS control for zeroizing side skidxe2x80x99 is found not to be practical and therefore, the optimal gain of the target rear-wheel steering angle relative to the front-wheel steering angle is set in trial and error fashion based on various vehicle dynamics derived from road test results.
In addition, though Japanese Patent Publication No. Sho. 60 (1985)-44185 discloses a rear-wheel steering angle control device in which a gain of a target rear-wheel steering angle relative to a front-wheel steering angle is made variable depending on vehicle speed, this reference remains silent as to how to set the gain of the target rear-wheel steering angle relative to the front-wheel steering angle. This means that there is no change in requiring an extreme amount of man-hours in setting the gain.
Thus, a need exists to provide a rear-wheel steer control device which drastically reduces the man-hours required for setting a gain of a target rear-wheel steering angle relative to a front-wheel steering angle.
The present invention has been developed to satisfy the request noted above and a first aspect of the present invention provides a rear-wheel steer control device which comprises a detecting means for detecting at least a vehicle speed, an actual front-wheel steering angle, and an actual rear-wheel steering angle; a target rear-wheel steering angle calculating means for calculating a target rear-wheel steering angle based on at least outputs of the detecting means; and a rear-wheel steering angle controlling means for controlling the actual rear-wheel steering angle detected by the detecting means to become equal or approximately equal to the target rear-wheel steering angle calculated by the target rear-wheel steering angle calculating means. Further, the target rear-wheel steering angle calculating means includes a variable coefficient inputting means for inputting variable coefficients and an inverse operation setting means in which a specific physical quantity which represents a vehicle motion is dealt with when a transfer function of the rear-wheel steering angle relative to the front-wheel steering angle is set and as a target transfer function a transfer function is employed which is obtained by multiplying specific parameters of a theoretical transfer function of the specific physical quantity relative to the front-wheel steering angle by the variable coefficients inputted from the variable coefficient inputting means. When an equivalent transfer function of the specific physical quantity relative to the front-wheel steering angle is represented by a transfer function of the rear-wheel steering angle relative to the front-wheel steering angle, a theoretical transfer function of the specific physical quantity relative to the front-wheel steering angle, and a theoretical transfer function of the specific physical quantity relative to the rear-wheel steering angle, the transfer function of the rear-wheel steering angle relative to the front-wheel steering angle is inversely calculated so that the equivalent transfer function becomes equivalent to the target transfer function. Moreover, the target rear-wheel steering angle calculating means calculates the target rear-wheel steering angle based on the transfer function of the rear-wheel steering angle relative to the front-wheel steering angle as calculated by the inverse operation setting means.
In the preceding description, as the xe2x80x98specific physical quantity which represents a vehicle motionxe2x80x99, vehicle-body slip angle, yaw rate, or lateral acceleration, etc. is available. The xe2x80x98theoretical transfer functionsxe2x80x99 are various theoretical formulas of transfer functions which are derived from xe2x80x98control theoryxe2x80x99 when an adequate vehicle dynamics model is assumed. The xe2x80x98specific characteristic parameterxe2x80x99 of the theoretical transfer function is each of the various parameters which appear in the calculated theoretical transfer functions. An example of the xe2x80x98specific characteristic parameterxe2x80x99 related to static characteristic is the steady gain of a specified physical quantity, while examples of the xe2x80x98specific characteristic parameterxe2x80x99 related to dynamic characteristic are natural angular frequency, damping or attenuation ratio, leading item, and so on.
In accordance with the first aspect of the present invention, the target rear-wheel steering angle calculating means calculates the target rear-wheel steering angle based on the transfer function of the rear-wheel steering angle relative to the set front-wheel steering angle, in such a manner that the theoretical transfer function of the specific physical quantity relative to the front-wheel steering angle is derived, in order to obtain transfer functions as target transfer functions by multiplying the specific characteristic parameters which appear in the theoretical transfer function with various variable coefficient inputted from the variable coefficient inputting means. Further, the equivalent transfer function of the specific physical quantity, relative to the front-wheel steering angle, is represented by three transfer functions: the transfer function of the rear-wheel steering angle relative to the front-wheel steering angle, the theoretical transfer function of the physical quantity relative to the front-wheel steering angle, and the theoretical transfer function of the physical quantity relative to the rear-wheel steering angle. The transfer function of the rear-wheel steering angle relative to the front-wheel steering angle is set by the inverse operation setting means.
Thus, the set transfer function of the rear-wheel steering angle relative to the front-wheel steering angle is represented in a form which includes the various variable coefficients inputted from the variable coefficient inputting means to be multiplied with the specific parameter, which makes it possible, when each of the various variable coefficients is tuned, to tune the gain of the rear-wheel steering angle relative to front-wheel steering angle. It is to be noted that tuning each of the various variable coefficients brings in tuning each of the specific characteristic parameters which appear in the theoretical transfer function of the physical quantity relative to the front-wheel steering angle, thereby enabling to tune each of the characteristic parameters for satisfying the driver""s feeling. Thus, when the gain of the rear-wheel steering angle relative to the front-wheel steering angle is tuned, the requirement is to tune only the various variable coefficients inputted from the variable coefficient inputting means while forecasting the change of vehicle dynamic characteristics due to changing each of the characteristic parameters, which makes it possible to remarkably reduce the man-hours required for setting the gain of the target rear-wheel steering angle relative to the front-wheel steering angle in trial and error fashion in road tests.
A second aspect of the present invention is to provide a rear-wheel steering angle control device to modify the structure of the first aspect, wherein the detecting means is capable of detecting a second specific physical quantity which represents the vehicle motion, and the target rear-wheel steering angle calculating means further includes a second specific physical quantity transfer function setting means in which a second specific physical quantity is dealt with and a transfer function of the second specific physical quantity relative to the front-wheel steering angle is calculated based on of the transfer function of the rear-wheel steering angle relative to the front-wheel steering angle which is calculated by the inverse operation setting means, a theoretical transfer function of the second specific physical quantity relative to the front-wheel steering angle and a theoretical transfer function of the second specific physical quantity relative to the rear-wheel steering angle, and a feedback control means for executing a feedback control in which a target second specific physical quantity calculated based on the transfer function of the second specific physical quantity relative to the front-wheel steering angle, which is calculated by the second specific physical quantity transfer function setting means, becomes equal to an actual second specific physical quantity which is detected by the detecting means, and wherein the target rear-wheel steering angle calculating means corrects the target rear-wheel steering angle by the feedback control means.
In the second aspect of the present invention, the second specific physical quantity, like the aforementioned specific physical quantity in the first aspect, can be in the form of vehicle-body slip angle, yaw rate, or lateral acceleration etc. The second specific physical quantity can be selected to be identical to the specific physical quantity.
Thus, correcting the target rear-wheel steering angle by providing the feedback control means makes it possible to set a much more precise target rear-wheel steering angle which is surely free from disturbances. In detail, the target second specific physical quantity, which is calculated based on the transfer function of the second specific physical quantity at the second specific physical quantity transfer function setting means, relative to the front-wheel steering angle is tuned by the various variable coefficients inputted from the variable coefficient inputting means and is input into the calculation which utilizes the transfer function of the rear-wheel steering angle, which is set at the inverse operation setting means, relative to the front-wheel steering angle. This second specific physical quantity is the target value which uses the second specific physical quantity for representing the expected vehicle dynamic characteristics resulting from tuning the various variable coefficients. On the other hand, the actual rear-wheel steering angle controlled by the rear-wheel steering angle control means to coincide with the target rear-wheel steering angle, which is calculated based on the transfer function of the rear-wheel steering angle relative to the front-wheel steering angle, may sometimes not coincide with the target rear-wheel steering angle due to disturbances. Such a difference causes the actual second specific physical quantity to change, which is detected by the detecting means. In addition, even if the actual rear-wheel steering angle is brought into coincidence with the target rear-wheel steering angle, the actual rear-wheel steering angle is not free from being changed due to disturbances. Thus, sometimes a deviation may appear between the target rear-wheel steering angle and the actual rear-wheel steering angle detected by the detecting means. However, providing the feedback control means for correcting the target rear-wheel steering angle to zero makes it possible to make a vehicle dynamic characteristic much closer to the expected vehicle dynamic characteristic, which is obtained when the various variable coefficients are tuned. Thus, setting a much more precise target rear-wheel steering angle is made possible. It is to be noted that as the second specific physical characteristic quantity employing one of yaw rate and lateral acceleration is preferable because real values are easy to detect using detecting means.